There's no reason not to do this since the majority of these users have access to both Chrome and Firefox - or a mobile device.
Podcasts lend themselves strongly to mobile. While there are other things on this website, a huge portion of the users are here to listen to and subscribe to the Ideas By Elliot Podcast.
There are pros and cons to any approach. Whether you think in terms of "80-20 Rules" or not, it is clear that far less than 20% of the visitors to this website will have any difficulty at all.
By embracing modern technology, we're able to roll-in new and exciting features very quickly. While itself not dependent on HTML 5 or CSS 3, the live stream feature took time to implement. Rather than struggling with obsolete layout and content techniques, we were able to very quickly implement our very popular live stream system. The Sponsor Portal, Artist Portal, and Guest Portal features were all able to be rolled out quickly because there was no need to test features on ancient software.
It's not as if there aren't enough choices for users. This is the list of browsers that this website is tested against: Mac/iOS Safari, Mac/Windows Firefox, Mac/Windows/Android/iOS Chrome as well as Internet Explorer 10, Internet Explorer 11, and Microsoft Edge. That's 11 different browsers. Pick one!